(1) Field of the Invention
This invention relates to a rubber composition and more particularly to a rubber composition containing a carbon black having selected properties and furnished with both a low heat build-up property and a high reinforcing property.
(2) Description of the Prior Art
Heretofore, a particle diameter (or a specific surface area) determined by an electron-microscopic method, a BET nitrogen adsorption method or an iodine adsorption method, and a structure given by a dibutyl phthalate absorption number have been important properties for carbon black used in rubber compounds. However, there has been a contradictory relationship between these typical properties, i.e., particle diameter and structure, and the properties of a rubber composition, especially low heat build-up property and high reinforcing property which are necessary for a rubber composition used in car tire carcasses and undertreads, in that the heat build-up is lowered and improved, while the reinforcing property is deteriorated as the carbon black particle becomes larger or the structure becomes smaller.
For this reason, it has been thought extremely difficult to effect lowering of the heat build-up of a rubber composition and at the same time furnishing a high reinforcing property.
As a result of a study made with attention paid to the fact that the interaction between a rubber component and compounded carbon black is influenced markedly by characteristics other than the particle diameter and structure of the carbon black, the applicant of this invention has found that an increase in tinting strength efficiently contributes toward increasing a reinforcing property and, on the basis of this finding, previously succeeded in developing a rubber composition which is furnished with both a low heat build-up and a high reinforcing property by imparting a selected property of tint strength of above 175 to a soft carbon black in a low heat build-up-imparting range having an electron-microscopic average particle diameter (dn) of about 31 millimicron, an iodine adsorption number of 40 to 60 mg/g, and a low DBP absorption number of 90 to 130 ml/100 g [Japanese Patent No. 1032175 (Japanese Patent Publication No. 13461/1979)].
However, this carbon black showed a behavior of heightening the Mooney viscosity of a rubber composition, depending on the condition for compounding, and has a problem of practice that smooth milling or extrusion is sometimes prevented.